1. Field of the Invention
The present invention relates to a switching power supply unit for connecting an output obtained by switching a direct current input voltage to an output winding of a power converting transformer.
2. Description of the Related Art
Hitherto, various types of switching power supply units have been proposed and provided for practical use. Many of them are of a type in which a direct current input voltage is switched by switching operation of a switch circuit connected to an input winding of a power converting transformer, and the switched output is connected to an output winding of the power converting transformer. A voltage appearing in the output winding in association with such switching operation of the switch circuit is rectified by a rectifier circuit, the rectified voltage is converted to a direct current voltage by a smoothing circuit, and the smoothed voltage is output.
In a switching power supply unit of this kind, an output rectifier device such as an output diode is connected in series with a power transmission line in the rectifier circuit. Therefore, to improve efficiency of the switching power supply unit, it is extremely effective to reduce a loss in the output diode.
To reduce a loss in the output diode, it is sufficient to use a diode having a small forward voltage drop. However, the diode having a small forward voltage drop has also a low reverse withstand voltage. Consequently, in the case of using a diode having a small forward voltage drop as the output diode, it is particularly necessary to suppress the reverse voltage.
In a switching power supply unit of this kind, a reverse voltage which has to be considered the most is a surge (spike) voltage caused by a parasitic element accompanying on/off operation of the switch circuit. The surge voltage is applied as a reverse voltage to the output diode. Hitherto, to suppress such a surge voltage, various attempts have been made.
For example, the applicant of the present invention has proposed a snubber circuit utilizing LC resonance in Japanese Patent Publication No. 3,400,443 (patent document 1). The snubber circuit can suppress the surge voltage to a predetermined voltage or less by utilizing LC resonance.
Each of U.S. Pat. No. 5,198,969 (patent document 2), U.S. Pat. No. 6,466,459 (patent document 3), and U.S. Pat. No. 6,650,551 (patent document 4) discloses a switching power supply unit having a circuit for suppressing the surge voltage.